Busy Tone Oscillator

TM 11-5895-1141-34

inverting operational amplifier stage U1B which in turn drives complementary emitter follower pair transistors Q2 and Q3.

This latter stage provides a signal to the HEADSET VOLUME control potentiometer R49 which is located on the front

panel. Resistor R49 is also fed with the sidetone signal current originating at the lamp brightener module assembly and

entering on pin P1-6. This sidetone signal level is nominally -25 dBm. The signal picked off by resistor R49 which can be

reduced as much as 20 dB when the control is fully counterclockwise, is then fed to two additional stages. One is Q4

which is a conventional class A transistor amplifier with a gain of unity when the transformer is terminated by the headset

or any 600 ohm load. The second stage fed by the VOLUME CONTROL resistor R49 is U1A, an inverting operational

amplifier stage which in turn supplies a signal current via resistor R35 and pin P1-7 to the recorder circuits, if used.

(5) Busy Tone Oscillator. Refer to figure FO-10. When the busy tone enable bus is driven low by the Q9 in the

selector module assembly, the oscillator circuit consisting of resistors R26, R27, R28, and capacitor C13, and transistor

Q5 is provided with a ground enabling it to operate. Q5 is a unijunction transistor and its negative resistance

characteristics are utilized here. When the voltage at the emitter (E) with respect to base No. 1 (B1) is less than about

65% of the supply voltage to base No. 2 (B2), no current flows through the emitter, and capacitor C13 is charged up by

current flowing through resistor R26. When the voltage reaches the 65% point (the standoff ratio) heavy conduction from

(E) to (B1) is initiated, discharging C13 very rapidly until the voltage on C13 drops to 2 v at which time the emitter to base 1

path reopens and C13 proceeds to recharge to about 17 v (65% point). The resultant voltage waveform between emitter

and ground is a sawtooth waveform with a steep falling slope. The period is determined by the time constant of resistor

R26 and capacitor C13 and the standoff ratio. A busy tone of approximately 800 Hz is produced when the enable bus is

grounded. Resistor R28 limits the peak current flow during discharge of capacitor C13 to safe values for the junction;

resistor R27 is provided to limit current flow from the supply to (B2) during these breakdown periods. The sawtooth

voltage is coupled to the input points of both the headset and speaker amplifiers by resistors R25 and R36 so that the busy

tone will be heard regardless of which amplifier is being used.

g. Speaker Amplifier Assembly. Refer to the schematic diagram, figure 2-11. This assembly is contained within

the audio unit enclosure. For this board, the actual component designation numbers are the basic numbers, shown on the

diagram, added to 1400. Thus, the input transistor shown as Q1 is actually Q1401. For brevity of discussion the basic

number will be used. Power is delivered to contacts F and H via resistor R101 (fig. FO-6) and returned via contacts K and

L. Capacitor C1 on the module, in conjunction with resistor R101 (fig. FO-6), filters the

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